1. Field of the Invention
The present invention is related to a microscope system having excellent operation performance, bright image and variable magnification.
2. Description of the Related Art
Single objective lens type stereomicroscope is known as a conventional example of a microscope with excellent operation performance and extendibility. FIG. 1 is a view showing an outline composition of an optical system of such single objective lens type stereomicroscope. The objective lens of this kind of microscope has a focal length in a range of about 200 to 40 mm, and is that configured to assure comparatively low magnification for observation and a long working distance.
However, in the conventional single objective lens type stereomicroscope, due to restrictions of the distance between two right and left optical paths from an afocal variable magnification optical system to an image forming lens, etc., a diameter at the entrance of the afocal variable magnification optical system is limited to ½ or less of the effective diameter of the objective lens. Especially, in a design where a large magnification ratio is achieved by the afocal variable magnification optical system, brightness of the image tends to be insufficient on the higher magnification side because of the lens diameter of the afocal variable magnification optical system as a limiting factor.
This is the same in such case that imaging is carried out by leading luminous flux which has transmitted either of the right or the left afocal variable magnification optical system to an image sensor.
When a fluorescence image is observed by using an objective lens having a considerably large effective diameter compared with the afocal variable magnification optical system, in combination with an illumination system and filters for fluorescence observation, sufficient brightness cannot not be obtained although a large field of view for observation is secured. Especially in an optical system having a large observation range in this way, a careless omission of a weak and minute fluorescence marker etc. easily occur.
Then, in order to increase brightness of an image in a specific magnification and a specific focal length, it is necessary to enlarge the numerical aperture NA of a lens. Enlarging the numerical aperture NA of a lens is equivalent to enlarging a diameter of luminous flux emergent from an objective lens.
Here, the diameter (in millimeters) of luminous flux emergent from the objective lens is expressed as follows:2·NA(ob)·FL(ob)  (1)where NA(ob) is a numerical aperture of an objective lens, and FL(ob) is a focal length (in millimeters) of the objective lens.
Generally a diameter of luminous flux emergent from the objective lens obtained from the condition (1) takes a value from about few mm to 20 mm in a microscope, and tends to be smaller as the focal length of the objective lens becomes shorter. Also in a stereomicroscope of a single objective-lens type, although the diameter of luminous flux emergent from the objective lens itself is about 40 mm or more, the effective diameter of the emergent luminous flux is about 20 mm, for it is limited by the effective diameter of an afocal variable magnification optical system.